What is Seam Allowance?

by | Dec 14, 2023 | All Documentation, Basic Pattern Making | 0 comments

When diving into the world of fashion design and pattern making, one term you’ll frequently encounter is seam allowance. This fundamental concept is crucial for both aspiring designers and seasoned professionals. But what exactly is seam allowance, and why is it so important in basic pattern making?
Ralph Pink points out the seam allowance on a basic block placed on a mannequin, elucidating a foundational concept in pattern making.

What is Seam Allowance?

Seam allowance refers to the area between the stitching line and the cut edge of the fabric. This extra fabric is essential for sewing a garment together. The seam allowance, or extra fabric outside of the sewing lines, allows for fit adjustments, provides room to sew the garment together without eating into the sew lines (which would cause the garment to be too small), and ensures that the seams are strong and durable since there is extra fabric to prevent fraying. Typically, seam allowances vary depending on the garment type, fabric, construction method, and whether it is either a basic block or finished pattern.

What is seam allowance? Basic block on dress mannequin

Its Use in Basic Blocks

Basic blocks are foundation patterns that form the base for all other finished patterns and garments. They are designed to fit the body accurately. Once a basic block’s fit on an individual or standard size has been perfected, it is then used to create hundreds of other finished patterns and garments, safe in the knowledge that the garment will fit. When fitting basic blocks, we use a variety of seam allowances to allow extra space during fittings.

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Side seam centre back seam allowance on a basic block. Basic pattern making

Side Seam & Centre Back (1.5 – 2.0 cm Seam Allowance)

We often add about 1.5 – 2.0 cm to the side seams and centre back of each basic block. This allows extra space during the fitting process. If the basic block has been drafted too tight, then the extra seam allowance can be used to add more room to the basic block, loosening the fit.

No seam allowance on the front of a basic block. Cut on fabric fold. What is seam allowance. Basic Pattern making

Centre Front (0 cm Seam Allowance)

Usually, the front basic block is cut on the fold. This means that the fabric is folded and the centre front of the basic block is placed on the fold of the fabric. When the basic block is cut on fabric, it is then unfolded to create a mirror image of the front basic block panel. Because of this, no seam allowance is added to the centre front of the basic block. If you wish to have a seam down the centre of the basic block, add it to the centre front of the basic block and cut it on fabric twice.

Shoulder and armhole seam allowance on a basic block. What is seam allowance. Basic pattern making

Shoulder & Armhole (0.5 – 1.0 cm Seam Allowance)

Shoulder: We usually add 1 cm to the shoulder. This doesn’t often need much alteration if the basic block has been drafted to match the customer’s bespoke measurements. 1 cm seam allowance is the standard for most finished patterns, but we’ll talk about this later.

Armhole: We usually add 1 cm to the armhole in case the armhole and sleeve need adjustments. However, if the sleeve and armhole are drafted well, 0.5 cm seam allowance is the best option. This removes excess bulk away from the armpit, allowing for a smoother, less bulky fit and improves arm movement.

Hem seam allowance on a basic block basic pattern making

Hem (2 – 2.5 cm Seam Allowance)

The hem could be considered seam allowance, although it is more often called the hem. This is because the finished edge of the garment is not a seam. Either way, we usually add around 2 – 2.5 cm seam allowance to the finished edge of the basic block, allowing room to lengthen or shorten the basic block for a perfect fit. When the hem is folded back on itself (up into the sample), it functions as a reinforced bottom edge to the garment. The hem can be as large or as small as you like; this usually depends on the design of the basic block or garment.

How to Apply Seam Allowance?

Seam Allowance can be applied or added to a basic block or pattern using one of two methods: either by hand using the traditional pattern making method, or it can be applied instantly using digital pattern making in Adobe Illustrator.

Traditional Pattern Making

Usually, a Pattern Master is used to apply seam allowance to a basic block or finished pattern by hand. A Pattern Master features 0.5 cm increments on the curves and edges of the Pattern Master ruler. These guides are used to add different allowances to various parts of the basic block. It’s a long and laborious process. The larger or more intricate the pattern, the longer it takes. Mistakes can also be made, causing messy lines.

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Digital Pattern Making

This process can be sped up. It’s almost instantaneous in digital pattern making techniques. We draft all our standardized basic blocks in Adobe Illustrator, which means they are fully editable basic blocks. Adding seam allowance becomes an effortless process, taking seconds, not hours. It’s also incredibly precise!

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Draft a Bespopke Basic Block

The Role of Seam Allowance in Finished Garments.

Standard Measurements

While seam allowances can vary with basic blocks for fitting purposes, there are some standard measurements commonly used in the fashion industry for finished patterns and garments. For most adult garments, 1 cm (3/8 inch) is typical for most parts of the garment or finished pattern, whereas the armhole and sleeve head will have 0.5 cm. However, for children’s wear, 0.75 cm (1/4 inch) is often used due to the smaller size of the garments.

Adjusting for Different Fabrics

Different fabrics may require different amounts. For instance, thicker fabrics might need a larger amount for ease of sewing, while delicate fabrics might benefit from a narrower seam allowance to reduce bulk.

Draft and adapt basic blocks in adobe illustrator. Digital pattern making
Draft and adapt basic blocks in adobe illustrator. Digital pattern making


Seam allowance is an important component in basic pattern making that ensures the functionality and style of a garment. It’s a basic, yet essential concept that forms the foundation of garment construction in fashion design. Knowing how much to use and where is something that needs to be understood and learned.

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